My table is the size they recommend. 3/4" plywood should be strong enough. But it is also important that it is flat and that the frame supporting it is rigid and doesn’t have any twist in the top plane where the plywood sets. Unless you get Baltic birch, the plywood won’t be very flat. I opted to go with two layers of 3/4" MDF that was glued and screwed together, which was less money that a single sheet of Baltic birch. I used the trim pieces for my spoil board.
I wouldn’t try to go much smaller. As mentioned above, you need space for running your wires and for the controller. I think it is important that the controller is close by and not covered up so you have fast access to the emergency stop button.
There are quite a few examples of table design if you just do a search. I posted mine here: W.I.P. CNC Table Design - #25 by Aiph5u
I have a pretty small shop but decided the space savings for a roll-around flip up wasn’t that great when you consider the footprint of the base. Several folks have created storage space with shelves or drawers. I decided to leave the space below my table open and I store my garbage cans and shop vac under it, so my lost space is actually somewhat minimal since those items were taking up floor space in the first place.
Hope this helps!